1. Field of the Invention
The present invention relates to the formation of a gate electrode on a semiconductor device, and more particularly to a photomask for forming a T-shaped gate structure on a FET and a process for fabricating the same.
2. Description of the Related Art
Recently, in order to carry the high modulation performance in high-speed devices such as a high electron mobility transistor (HEMT) being applicable mainly to a satellite broadcasting receiver, a high-speed logic circuit, a power module, MMIC and the like, not only short gate lengths but also large cross-sectional areas for allowing massive current flow are needed. To comply with these conditions, a T-shaped gate structure that looks like the letter "T" has been used.
Meanwhile, in case the T-shaped gate is formed by a photolithography, it is not sufficient to form a gate pattern having a width of less than 0.25 microns due to the limitation of resolution of the photolithography per se.
Thus, a T-shaped gate structure has been generally formed using an electron beam lithography, in which two layers of electron-beam resists, each having a different sensitivity, is deposited and then patterned to form the T-gate structure.
That is, the bottom layer is a low sensitivity polymethyl methacrylates (PMMA) for an electron-beam while the top layer is formed by a high sensitivity methyl methacrylate (MMA) for the electron-beam.
This typical technique will be described in detail in conjunction with the accompanying drawings.